Lab #2
Soil Color and Texture
Soli Color and Texture- By Feel.
Soil Color
Color is an easily determined soil characteristic. Even though soil color directly effects
only the absorption of solar radiation, soil color can provided valuable information about
other soil properties. The color of surface horizons is often related to their organic matter
content. In addition, differenced in soil color is one of the properties that can be used to
differentiate soil horizons.
Munsell color notation system.
Soil color is most conveniently measured by comparison of the soil color with colors on a
color chart. The collection of color charts generally used with soils is a modified version of
the collection charts appearing in the Munsell Book of Color and includes only that portion
needed for soils. The Munsell notation identifies color by the use of three variables, Hue,
Value, and Chroma.
Hue is the dominant spectral (rainbow) color; that is whether the color is yellow, red, green
or mixtures such as yellow-red. Mixtures are identified numerically according to the amount
of yellow or red used to produce the mixture. 5YR is an equal mixture of red and yellow. As
the number increases the amount of the first letter color ( Y, yellow) increases and as the
number decrease the amount of the second letter color (R, red) increases.
Value and Chroma are term that refers to how the hue is modified by the addition of grey to
the pure color (hue). Value is a property of the grey color that is being added to the hue. A
particular grey (value) is made by mixing a pure white pigment (10) with a pure black
pigment (0). If equals amounts of white and black pigments are mixed the values is equal to
5, if more black than white pigment is used then the value is less than 5, if more white than
black pigment is used then the value is greater than 5. Chroma is the amount of pure hue that
is mixed with a grey of pure hue that is mixed with a grey of a particular value to obtain the
actual color. A chroma of 1 would be made by adding one unit of pure hue to certain amount
of grey, a chroma of 5 would contain 5 units of pure hue to that amount of grey. The lower
the chroma the greyer the color.
Determination of soil color.
The nomenclature for soil color consists of two complementary systems: color
names and the Munsell notation of color. Color names are less precise, but convey a
general concept of the color of the soil. Munsell notation is more precise and is
standardized so that soil scientists in different countries will have no difficulty in
communicating information about soil color.
The Munsell soil color charts are set up so that each page is a separate hue, for
example 10 YR or 5R. The hue is given in the upper right hand corner of the chart (page).
An actual chart is a collection of color chips of constant hue arranged by value and
chroma. Rows of color chips are for a constant value, with chromas increasing from left
to right. Columns are for a constant chroma with values decreasing from top to bottom.
Soil color is determined by matching a moist soil sample with the appropriate
color chip. Because a given color chart is not a collection of all possible values and
chromas of a given hue, the match is often less than perfect. It is generally possible to
describe one chip that I very close in color to the moist soil. The major difficulties
encountered in using the soil color charts are 1) in selecting the appropriate hue card 2)
determining colors that are intermediate between hues and 3) distinguishing between
values and chromas when chromas are strong (high numbers).
Once the appropriate color chip has been selected, the hue, value and chroma are
recorded. The notation 5 YR 5/4 is for a soil with a hue of 5 YR, a value of 5 and a
chroma of 4. The corresponding color name for 5 YR 5/4 is reddish brown. If expression
of soil color more precise than whole numbers for values and chromas is desired,
decimals are used, never fractions. Generally color is only giving in integer (whole)
numbers. Reproducible measurements of soil color can be obtained at two moister
contents: moist (field-capacity) and air0dry. In most soil descriptions, unless otherwise
stated, colors are given for moist soils.
Interpretation of soil color.
Soil color is basically due to: 1) The Organic matter imparts a brown to black
color to the soil. Generally the higher the organic matter content of the soil the darker the
soil. 2) The oxidation status of the iron compounds in the soil. In the lower horizons,
where the soil minerals are not coasted with humic substances, the color of the soil
minerals will predominate. In better drained and, hence, well-aerated soils Fe (III)
minerals give soils a red or yellow color. In more poorly drained and, hence, poorly
aerated soils the iron minerals are reduced and the neutral (grey) colors of Fe (II)
minerals and the other soil minerals predominate.
Soil organic matter. The organic
matter content of soils can be estimated
from the Munsell color of a soil (J.D.
Alexander, Univ. of Illinois AG-1941). The
most accurate estimates are obtained with
medium and fine texture mineral soils. Soils
with greater than 50% sand and less than
10% clay usually contain less organic
matter than predicted. The performance of
many herbicides is influenced by adsorption
onto soil organic matter. Estimation of soil
organic matter from soil color can be
helpful in selecting herbicides and
determining application rates. Appropriate
rates maximize weed control, while
minimizing crop damage and potential
environmental effects.
Table 1-1 Relationship of soil color to soil organic matter content
Munsell Notation
Moist color
10YR 2/1
10YR 3/1
10YR 3/2
10YR 4/2
10YR 5/3
Percent Organic Matter
Range
3.5 – 7.0
2.5 – 4.0
2.0 – 3.0
1.5 – 2.5
1.0 – 2.0
Avg
5.0
3.5
2.5
2.0
1.5
Soil drainage classes.
The drainage class of a soil can be determined from the colors and
the color patterns in the soil’s lower horizons (subsoil). The red color of soils can be
determined from the colors and color patters in the soil’s lower horizons (subsoil). The red
color of soils is generally related to the presence of unhydrated iron (III) oxides, although
manganese dioxide and partially hydrated iron (III) oxides may also contribute red colors.
The red colors may be inherited from the parent material or developed by the oxidation of
iron material during soil weathering. Red colors are stable only in soils that are well-aerated.
The yellow color of soils is largely due to the presence of hydrated iron (III) oxides. Soils
with yellow colors tend to occupy wetter landscape positions than associated red soils, this
result in the hydration of the iron (III) oxides. Grey and whitish colors of soils are caused by
several substances, mainly quartz, kaolinite and other clay minerals, calcium and magnesium
carbonates (limestone) and reduced iron compounds. The greyest colors (chromas less than
1) occur in permanently saturated soil horizons, these soils often have a bluish appearance.
Soil horizons may be uniform in color or may be streaked, spotted, variegated or mottled.
Local accumulations of carbonates or organic matter can produce a spotted appearance.
Streaks or tongues of color may result from the downward movement of clays, organic
matter and/or iron oxides. Mottling is often associated with fluctuating water tables creating
changes in soil drainage and aeration and, hence, mixture of red, yellow and grey colors.
Natural drainage classes.
Very poorly drained
Soils on level or depressional areas frequently ponded with
water. Black or dark grey surface horizons, light grey
colored horizons immediately under the surface horizons.
Poorly drained
Soils having high water tables or slowly permeable layers
in the profile. Mottling occurs immediately under the
surface horizons. Lower horizons light grey in color.
Somewhat poorly drained
Usually has mottles between 25-45 cm from soil surfaces.
Light grey colors not presence. Except deep into parent
materials.
Moderately well drained
Usually mottling first occurs 45-75 cm from soil surface.
Well drained
Usually fee from mottles. Has uniform brow and yellowish
or reddish brown colors in subsoil. If mottles occur they are
below 75 cm depth.
Soil Texture.
Soil textures refers to the percentage of sand, silt and clay particles in a given
mass of soil. Particles greater than 2 mm in diameter are removed from the soil by
sieving and are excluded from the textural determination. The presence of larger particles
is recognized by the use of modifiers such as gravelly, cobbly, stoney, cherty, slaty, or
shaly based on the size and composition of the larger particle.
Sand
Particles ranging in size from 2 mm to .05 mm in diameter.
Silt
Particles ranging in size from .05 mm to .002 mm in diameter.
Clay
Particles less than .002 mm in diameter.
Sand, silt, and clay are size separates that include all mineral particles in a specific size range
regardless of composition or mineralogy.
Textural classes
Soils normally are mixture of sand, silt, clay, larger particles and organic matter. The
larger mineral particles and the organic matter are removed before texture analysis. Soils are
then classified into different textural classes based on the percentage of sand, silt, and clay in
the soil. Textural classes share similar physical and chemical properties, that is similar water
holding capacities or cation exchange capacities, ect.
Determination of a soils textural class is best achieved using a textual triangle. A textural
triangle is constructed so that the bases represent 0% sand or silt or clay and the
corresponding apexes represent 100% sand or silt or clay
Laboratory directions
1. Determine the soil color of the soil samples provided by the instructor.
Procedure for determining soil color:
a. Select the soil sample to be described.
b. Moisten the sample so that no free water is present nor is it glistening in the
light.
c. Determine the hue of the sample by comparison with pages in the Munsell
color book. Begin with the 10YR hue as a starting point.
d. With good light coming over your shoulder compare the soil’s color with the
color chips until you find the best match possible. You may have to select
another hue (page) if you have difficulty finding a match.
e. Record the Munsell color notation (hue value/chroma) and the soil name.
2. Determine the texture of the unknown samples provided by the instructor using the chart
provided.
Name:__________________________________
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Soil Color Ribbon Length
Soil feel
Soil Texture
Name ___________________
RIBBON TEXTURE TEST
(Excerpt from “The Nature & Properties of Soils”)
The first, and most critical, step in the texture-by-feel method is to knead a walnut-sized sample
of moist soil into a uniform puttylike consistency, slowly adding water is necessary. This step may take a
few minutes, but a premature determination is likely to be in error as hard clumps of clay and silt may
feel like sand grains. The soil should be moist, but not quite glistening. Try to do this with only one
hand so as to keep your other hand clean for writing in a field notebook.
While squeezing and kneading the sample, note its malleability, stickiness, and stiffness, all
properties associated with the clay content. A high silt content makes a sample feel smooth and silky,
with little stickiness or resistance to deformation. A soil with a significant content of sand feels rough
and gritty, and makes a grinding noise when rubbed near one’s ear.
Get a feel for the amount of clay by attempting to squeeze a ball of properly moistened soil
between your thumb and the side of your forefinger, making a ribbon of soil. Make the ribbon as long
as possible until it breaks from its own weight.
Interpret your observations as follows:
1. Soil will not cohere into a ball, falls apart: sand
2. Soil forms a ball, but will not form a ribbon: loamy sand
3. Soil ribbon is dull and breaks off when less than 2.5 cm long and
a. Grinding noise is audible; grittiness is prominent feel: sandy loam
b. Smooth, floury feel prominent; no grinding audible: silt loam
c. Only slight grittiness and smoothness; grinding not clearly audible: loam
4. Soil exhibits moderate stickiness and firmness, forms ribbons 2.5 to 5 cm long, and
a. Grinding noise is audible; grittiness is prominent feel: sandy clay loam
b. Smooth, floury feel prominent; no grinding audible: silty clay loam
c. Only slight grittiness and smoothness; grinding not clearly audible: clay loam
5. Soil exhibits dominant stickiness and firmness, forms shiny ribbons longer than
5 cm, and
a. Grinding noise is audible; grittiness is dominant feel: sandy clay
b. Smooth, floury feel prominent; no grinding audible: silty clay
c. Only slight grittiness and smoothness; grinding not clearly audible: clay
A more precise estimate of sand content (and hence more accurate placement in the horizontal
dimension of the textural class triangle) can be made by wetting a pea-sized clump of soil in the palm of
your hand and smearing it around with your finger until your palm becomes coated with a soup like
suspension of soil. The sand grains will stand out visibly and their volume as compared to the original
“pea” can be estimated, as can their relative size (fine, medium, coarse, etc.).
It is best to learn the method using samples of known textural class. With practice,
accurate textural class determinations can be made on the spot.